The present invention relates to filtering particles out of liquids and relates in particular to a method and an arrangement for filtering particles out of a liquid by means of hydrostatic pressure acting on a filter member which is submerged in the liquid in a tank.
For filtering solid particles out of liquids, a large number of different filtering principles are known within the processing industry, which are adapted to the types of particles to be filtered and the desired degree of filtering of the filtrate. As the cost of many filter aids is relatively high, various filter constructions and arrangements have been developed, in which the filter member in the form of, for example, a drum filter, a sand filter, a coating filter etc., is self-cleaning in some way, for example by back-flushing or by means of scrapers, brushes or the like, which clean the filter surface of filter cakes accumulated thereon. A disadvantage of such self-cleaning filters, however, is that they create waste which has to undergo further treatment In sand filters, for example, the waste flow, that is to say the flushing water, accounts for roughly 5-10% of the total flow. Cleaning of the filter surface by means of mechanical aids, such as scrapers and brushes, usually takes place in contact with air, which contributes to clogging of the filter surfaces.
EP-0 848 977 A2 describes a method and arrangement for filtering particles out of a liquid, in which the liquid containing particles is supplied to a cleaning tank, the liquid being made, by means of hydrostatic liquid pressure prevailing in the tank, to flow through a particle-collecting filter member on the shell surface of a drum rotating continuously in the tank after which cleaned liquid inside the drum is drained off via an outlet The drum operates with roughly ⅔ of its height submerged in the liquid, and the filter on the inside of its shell surface is cleaned continuously by means of back-flushing by compressed air from inside the drum during its rotation, filter cleaning taking place in air above the level of the liquid.
One object of the present invention is to eliminate said disadvantages of known filtering systems and propose a method and an arrangement for filtering particles out of liquids, in which the self-cleaning filter system does not consume filter aids or produce other waste from the liquid mixture than the sludge which is collected by the filter member, and in which the cleaning system operates below the liquid surface so as to avoid clogging of the filter member and to increase the filtering capacity of the cleaning arrangement.
To achieve this object, the method and the arrangement for filtering according to the present invention are characterized by the features indicated in the patent claims below.